The invention relates to a combination of a feedthrough element for an electric high-frequency signal and a probe for guiding said high-frequency signalxe2x80x94as it is, for example, generated in a level metering device and evaluated after reflection at a filling product surface to be monitoredxe2x80x94to the filling product surface and back from there. The feedthrough element comprises a guiding element, into which the electric high-frequency signal is to be fed at an inlet point, and which transmits the electric high-frequency signal to the probe at an outlet point. Moreover, the feedthrough element comprises a one-part or multipart mechanical carrier element. A one-part or multipart insulation is present between the carrier element and the guiding element. The invention further relates to level metering devices (TDR devices) working on the principle of transit or propagation time measurement of guided electromagnetic waves and being equipped with a feedthrough of the aforementioned type.
For level metering operations, measurement systems are used, which determine the distance from the filling product on the basis of the measured transit time of electromagnetic waves from a level metering device mounted in the receptacle cover to the surface of the filling product and back. The required level can be calculated when the receptacle height is known. Such sensors known under the technical designation of level radar, are all based on the property of electromagnetic waves of propagating within a homogenous non-guiding medium at a constant speed, and of being at least in part reflected at the boundary surface of various media. Each boundary layer of two media having various dielectric constants, generates a radar echo upon impingement of the wave. The greater the difference between the two dielectric constants, the more the impedance level of the wave propagation changes, and the stronger is the echo to be observed.
Various radar principles are known for determining the required wave propagation time. The two mainly used methods are the pulse-time delay method (pulse radar), for one, and the frequency-modulated continuous wave method (FMCW radar), for another. The pulse radar uses the pulse-shaped amplitude modulation of the wave to be emitted, and assesses the direct time interval between emission and reception of the pulses. The FMCW radar determines the transit time in an indirect way by emitting a frequency-modulated signal and by differentiating between emitted and received instantaneous frequency.
Apart from the various radar principles, various frequency ranges of the electromagnetic waves are used, as well, depending on the respective application. Thus, for example, pulse radars exist having carrier frequencies in the range from 5 to 30 GHz, and in addition likewise those working in the base band as so-called monopulse radar without carrier frequency.
A series of methods and devices is moreover known for guiding the electromagnetic wave to the surface of the filling product and back. Thereby, the basic difference is made between a wave radiated into the space and a wave guided through a line. A level measuring apparatus in which microwaves are fed via a coaxial line into an antenna meant for radiating electromagnetic waves is known from EP 0 834 722 A2. Here, the antenna is configured in two parts. One antenna part in the form of a solid cylinder consists of a dielectric material and is shrouded by a metal sleeve. The microwave is fed in at one end of the solid cylinder of a dielectric material, while at the other end ensues the transmission to the radiating end of the antenna. The metal sleeve extends over the antenna zone configured as a solid cylinder and being present in the zone of a neck of a vessel containing the filling product. This antenna structure, in particular the configuration within the neck of the vessel, therewith constitutes a filled waveguide for transferring the high-frequency signal or the wave into the antenna zone meant for radiation. This structure has the effect that the antenna, in the zone of the attachment of the measuring apparatusxe2x80x94hence in that part of the antenna situated in the zone of the neckxe2x80x94does not transmit microwaves and does not receive reflected microwaves, respectively. To avoid an impedance leap at that end of the metal sleeve facing the radiating antenna, the sleeve end is bevelled.
From EP 0 922 942 A1, a filling level measuring device with a radiating antenna is likewise known to work with microwaves. Here, the microwave fed through a coaxial cable, is introduced into an end element, which is configured with a cone at the antenna side. Following same, there is an insert of a dielectric material comprising a recess in the end element corresponding to said cone. Then from this insert of dielectric material ensues the further transmission of the microwave to the radiating antenna parts. A higher portion of ceramic is featured in the direction facing away from the antenna than in a section arranged in the transmitting direction facing the antenna in order to achieve a quasi-continuous transition without having substantial impedance leaps.
Radar sensors exhibiting a completely different structure with respect to the feedthrough and the signal guidance, which guide the electromagnetic wave through a line (probe) to the reflection place and back, are also designated as TDR (time domain reflectometry) sensors. These sensors, as compared to those which freely radiate high-frequency waves, have a substantially lower attenuation of the reflected echo signal, since the power-flow only ensues in the constricted area in the environment along the conducting waveguide. Moreover, interfering echoes from within the receptacle, originating, for example, from the reflections of the wave at receptacle components (stirrers, tubes), and which complicate the identification of the very one echo from the surface of the filling product with freely radiating sensors, are avoided to a large extent with sensors having guided waves. This leads to the fact that level metering with guided electromagnetic waves is to a large extent independent of the receptacle construction and moreover of the product properties of the filling product or other operational conditions (e.g. dust, angle of the bulk good), and therefore leads to highly reliable measurement results.
All known leads usual for high frequency can be used as the waveguides for guiding the wave, in which the wave penetrates at least in part the medium surrounding the metallic leads or is enclosed by same. Due to their simple mechanical structure and their suitability for any filling products, i.e. bulk goods and liquids, the single-wire line or single-wire probe in particular is often used in the level metering technology. In its configuration as a rod or cable probe, it is above all insensitive to deposits and adherences of filling products. In DE 44 04 745 C2, a level metering sensor including such a probe is described as an example.
An important aspect of the TDR level metering sensors having single leads, is the input of the measurement signal from the electronic unit into the probe. Thereby, it is important that the path leading from the electronic unit to the probe, does not contain any major impedance leaps for the guided wave. Since a part of the wave is reflected by every discontinuously changing line impedance, this reflected portion, for one, is no longer available for the measurement purpose, hence the reflection at the surface of the filling product, thus causing an amplitude loss of the echo generated there. Moreover, additional interfering echoes are generated by the wave reflecting at possible line impedance variations between the electronic unit and the probe, which complicate the identification of the filling product reflection to be evaluated. This is due, in particular, to the fact that the echo interfering at the irregularity between the receptacle feedthrough and the probe extends in each case depending on the bandwidth used of the measurement signal over a distance area directly following said irregularity. With the low echoes from the filling product surface to be measured and a high interfering echo from the impedance leap at the beginning of the probe, it becomes impossible to detect and precisely measure levels reaching the upper end of the probe. Therefore, in all known TDR sensors, a minimum spacing between the filling product and the feedthrough for the signal through the receptacle wall is provided, which should not be fallen short of. Usually, this is at about 30 cm.
The line path between the electronic unit and the probe in level metering sensors consists in all cases of the mentioned feedthrough and, in addition, in most cases of a coaxial cable establishing the connection to the printed board whereon the electronic circuit for the generation of the transmitted signal and the evaluation of the reflected signal is assembled. The coaxial cable can be dispensed of in special cases when the printed board comprises a direct electrical and mechanical connection to the feedthrough.
The feedthrough serves for guiding the measurement signal from the sensor fixed at the outside of the filling product receptacle to the probe extending within the receptacle. Moreover, it has to supply a mechanical support for the probe. For this purpose, it is usually provided with a carrier element of metal, which can be secured to the receptacle, e.g. in a cover opening of same, and which mechanically supports a guiding element guiding the wave. An insulating element is disposed between the carrier element and the guiding element so as to avoid short-circuits. The guiding element connects, for one, the coaxial cable usually leading to the electronic unit and, for another, the probe mounted within the receptacle.
Feedthroughs for single-line probes are usually structured coaxially, i.e. the guiding element is coaxially surrounded by the insulating element and the carrier element. This basic structure, hence, can be technically configured in various ways, so as to comply with determined requirements such as the sealing of the receptacle atmosphere, pressures resistance, reception of high tensile forces at the probe, high temperature and resistance to aggressive receptacle atmospheres. Apart from the mechanical requirements made on the feedthrough, the electric requirementxe2x80x94as already mentionedxe2x80x94of a wave guidance without major impedance leaps has to be observed. This requirement can be complied with for the coaxial line within the feedthrough. Examples of such solutions for electrically as well as mechanically suitable feedthroughs can be found in EP 0 773 433 A1, EP 0 780 664 A2 and WO 98/25 109. All of the therein described feedthroughs furnish indications as to how the line impedance is mostly to be kept constant within the feedthrough.
No solutions can be found in these documents for matching the inevitable impedance leap between the coaxial feedthrough and the probe (also called single conductor) following same. The mentioned impedance leap is normally very distinctive and thereby particularly disturbing. The reason for this is the fact that the line impedance of a single conductor is in the order of 300xcexa9. With coaxial conductors, the line impedance results from the relationship of the outer conductor D to the inner conductor d and the impedance of the dielectric constants of the interposed insulating material. The more important the relationship D/d and the smaller the dielectric constant, the more important the impedance becomes. Dimension D of the outer conductor is in practice delimited towards the upper side by usual receptacle openings, dimension d of the inner conductor is delimited towards the lower side by the necessary mechanical stability of the guiding element. In toto, the line impedance is hence restricted by the pre-given delimitations of the mechanical dimensions towards higher values.
Impedance values which are simple to realize for coaxially structured feedthroughs are between 50xcexa9 and 100xcexa9, and are usually so dimensioned that they carry on the impedance of the coaxial cable connecting them to the electronic unit. This means that the impedance of the coaxial feedthrough is often close to the standard values of 50xcexa9 or 75xcexa9. The result from this consideration is an impedance leap at the connecting point feedthrough-single conductor of significantly more than the factor 2. The hitherto known improvement of the line matching from the feedthrough impedance to the impedance of the single conductor is, for example, described in the already mentioned DE 44 04 745 C2. By means of a matching horn following the feedthrough, the impedance level does not pass over discontinuously but passes over rather continuously from the lower value of the feedthrough to the higher value of the single conductor. The disadvantage of this solution is the space required by the horn inside the receptacle and the risk of the filling product adhering to the inside of the horn, as well as the possible damaging influences of the receptacle atmosphere on the horn.
The invention is based on the technical problem of providing a feedthrough for high-frequency signals in a TDR level metering device improved with respect to interfering echoes.
This technical problem is solved in a preferred embodiment by an inventive combination composed of an impedance-featuring feedthrough element for an electric high-frequency signal within a TDR level metering device, and an impedance-featuring probe for guiding the high-frequency signal to the filling product surface and back from there. The feedthrough element comprises a guiding element into which the electric high-frequency signal is to be fed at an inlet point, and which transmits the electric high-frequency signal to the probe for guiding said high-frequency signal, a one-part or multipart mechanical carrier element, and a one-part or multipart mechanical insulation present between the carrier element and the guiding element. The impedance of the feedthrough element and the impedance of the probe following the outlet point are substantially matched to each other.
An inventive combination of the initially mentioned kind is characterized in that the impedance of the feedthrough and the impedance of the probe meant for guiding and not for radiating the high-frequency signal, are for the first time substantially matched to each other at the outlet point. In contrast to prior art, the impedances at the outlet point of the feedthrough hence are given attention for the first time, and by the impedance matching in this zone, hitherto occurring interfering reflections are avoided to a large extent or at least reduced.
The novel impedance matching can be achieved, for one, in that the impedance within the feedthrough is increased by constructional measures to the higher impedance of the probe. Such inventive embodiments comprise, for example, dimension variations of various components inside of the feedthrough (cf. e.g. FIGS. 2 and 3). For another, it is yet possible, to reduce the higher impedance of the probe to the lower impedance of the feedthrough. This is achieved by adding appropriate components close to the outlet point, such as it is shown, for example, in various variations in FIGS. 6 through 8. From FIG. 9, it can moreover be seen that the two solution principles can also be combined.
According to the first solution principle, the impedance at the outlet point for being matched to the impedance of the probe following the outlet point, hence is substantially higher than the impedance at the inlet point. The impedance at the outlet point therewith is supposed to be matched to the probe impedance so as to avoid impedance leaps having the above-described disadvantages. An improvement with respect to prior art is achieved as soon as the impedance of the probe is not higher than 1.5 times the impedance of the feedthrough at the outlet point, a fact, which in the sense of the invention, is to be understood as an essential matching of the impedances. This means that, for example, under taking the aforementioned conditions of prior art as a basis, the impedance of 50xcexa9 at the inlet point is increased to an impedance at the outlet point of 200xcexa9, whereby interfering reflections are substantially reduced as compared to prior art.
The invention is based on the idea of preventing for the first time an undesired impedance leap from occurring for the first time as opposed to the prior art which works with space-consuming devices connected downstream of the feedthrough, but by providing for an impedance matching in the very feedthrough at the transition (outlet point) from the feedthrough to the probe situated outside of the feedthrough. This can be performed by a suitable selection of individual component materials, a novel shaping of single or plural feedthrough components or also by elements which can be simply integrated into the feedthrough and having an impedance matching effect, such as, for example, a discrete resistor, a wave-attenuating element or a high-frequency transformer. Of course, combinations of two or more of the mentioned solution examples are possible, too.
An embodiment of the invention consists in realizing in a coaxially structured feedthrough, a match between the impedance of the connection to the electronic unit and the impedance of the probe by continuously modifying the line impedance. This can, for example, be realized by continuously modifying the ratio between the inner diameter of the carrier element and the diameter of the guiding element, in each case relative to a cutting plane perpendicular to the direction of the wave propagation. Thus, a continuous impedance modification from 50xcexa9 to 300xcexa9 is, for example, possible. The longer the zone of the continuous impedance modification can be construed, the more low-reflection is its effect. In the ideal case, the wave is guided from one end of the feedthrough to the other without major reflections and is not faced with major impedance changes at the connecting point to the probe, either. The variation of the diameter ratios realized continuously or, alternatively, in several stages can be achieved by a tapered shape of the guiding element, by a conical inner contour of the carrier element or by a combination of these two possibilities.
With a required minimum diameter of the guiding element and a receptacle-relative restricted outer diameter of the carrier element, however, this solution is only usable under certain conditions, since in this case, any arbitrarily high impedances cannot be realized within the feedthrough.
A further embodiment of the invention without the above-mentioned restriction consists in structuring the feedthrough not purely coaxially but to realize within the carrier element of metal a so-called two-wire line. By continuously varying the thickness and the spacing of the additional second guiding element, as well as by restricting the length of the second guiding element to the constructional length of the feedthrough, a continuous impedance increase of the circuit can be achieved despite a restricted cross section area. In this construction, only the probe having all the known advantages of this simple probe protrudes into the receptacle as before. The impedance of a two-wire line can be realized relatively high, e.g. about 250xcexa9, in particular when the latter is structured asymmetrical (i.e. the wire diameters are different) on a less loaded cross section area. Likewise, however, on the same restricted cross section area impedances of  less than 100xcexa9 are possible with the two-wire line.
A further embodiment of the invention consists in improving through an attenuation of the wave within the feedthrough the impedance matching of the coaxial line, for example, having a small cross section area, to the single line protruding into the receptacle.
This lossy matching which is in principle known, can be realized for the high-frequency feedthrough either by building-in a discrete ohmic resistance or by using a wave-attenuating material as the line dielectric. As a preferred example of realization, a material having a fine distribution of conductive pigments within a filler material is to be mentioned, such as, for example, fine graphite powder admixed to a teflon mass. By the volume, the shape and the conductivity of the wave-attenuating material, the desired impedance matching may thereby be optimized. This solution principle hence does not basically reside on a first parallel connection of the impedance of the probe and the ohmic resistor and/or the resistor formed by the wave-attenuating material. This first parallel connection is in turn connected in parallel with the impedance of the coaxial line of the guiding element and the carrier element. In summary, a parallel connection of the impedance of the probe, the discrete ohmic resistor and/or the wave-attenuating material and the impedance of the coaxial line is created in the zone of the outlet point.
The wave-attenuation within the feedthrough of course causes an amplitude reduction to occur with the filling product reflection to be evaluated, but as compared thereto, the interfering echo at the connection point between the feedthrough and the single line is reduced in a stronger manner, so that in toto a more favorable ratio between the useful echo and the interfering echo is achieved. This method of lossy matching may be applied in an advantageous manner to the coaxial feedthroughs known from prior art as well as to the above-described feedthrough having a two-wire line.
A further embodiment of the invention consists in matching the relatively low impedance of the waveguide within the feedthrough and the relatively high impedance of the single-line probe to each other by means of a high-frequency transformer. Such an impedance transformation by means of a transformer is in principle known. The impedance from the input to the output of the transformer changes in a square-law manner relative to the voltage transmission ratio or the winding ratio. In the case of the feedthrough to be optimized, the transformer, however, has to be mounted at the location of the existing impedance leap, i.e. at the transition of the feedthrough to the single line. A solution for this is to support the single line insulated within the feedthrough and to connect the transformer with the starting end of the metallic single line close to the end of the feedthrough. By modifying the transformer""s winding ratio, a matching of various, theoretically arbitrary input and output impedances can be achieved.
In this embodiment of the invention, the impedance leap including the therewith associated disadvantages is so to speak xe2x80x9cpreventedxe2x80x9d from occurring by the high-frequency transformer. In contrast to the embodiment as per FIG. 2, in which the matching of the impedances at the outlet point is achieved in that the impedance within the feedthrough is substantially increased, and namely towards the higher impedance of the probe, here, the impedance within the feedthrough has not to be substantially changed. Now, a mutual impedance matching is xe2x80x9cenforcedxe2x80x9d by the high-frequency transformer.